1. Field of the Invention
This invention relates to the continuous transport of materials by conveyor belts. In particular, this invention relates to high-performance conveyor belts which are formed into a trough and which are supported by flexible elastic rollers.
2. Background Information
Conveyor belts are often used for the transport of materials in bulk, and quite often those conveyor belts need to be able to transport material of very varied dimensions, or "whatever comes along". For the transportation of materials in quarries, for example, conveyor belts of various lengths are often used, which lengths can be from several meters to several kilometers long in one continuous segment. Continuous belts for use in such situations often are constructed with a reinforcement casing which can be made of textile or metal, and such belts are generally supported at various intervals by small-diameter rotating rollers mounted on roller bearings. The spacing interval for such rollers is generally determined as a function of the load which the belt will carry, and of the tension exerted on the belt at any given point.
U.S. Pat. No. 2,271,166 to Weiss discloses one example of support rollers for supporting a conveyor belt. U.S. Pat. No. 2,271,166 essentially relates to an easily replaceable roller packing, which packing can be replaced without expensive disassembly of the entire roller structure. The separate rollers form the contact surface of each of the rigid rollers, while the rigid rollers support the separate rollers by means of several removable rims.
It is known that the formation of a conveyor belt into a trough-shaped cross-section can result in an increase in the capacity of the material transported on conveyor systems. The prior art of troughing a conveyor belt consists of mounting two rigid cylinders at a significant angle from the horizontal so that the two cylinders are able to idle on their shafts. These two cylinders are generally oriented symmetrically in relation to the axis of the belt.
To clarify the terminology which is used in this application, the term "roller" will generally be used for individual elementary support cylinders, and the term "rigid support" will be used to refer to the fastening parts for supporting the roller bearings and the set of two or three rollers, essentially contained in the same vertical plane, orthogonal to the longitudinal direction of the belt.
When two rollers are used to support and trough a belt, the two rollers are generally arranged in a "V"-shape in a vertical plane. This arrangement produces a small-radius curvature in the belt at the place where the two rollers meet. The load also accumulates in this area, and this causes local fatigue to the reinforcement casing. This fatigue is most noticeable in the zones where the longitudinal slope changes.
It has become apparent that it is necessary to use a third horizontal roller for supporting the center of the load, thereby forming a trough having a trapezoidal shape. Use of this third roller in the center shifts the area of curvature, which has a lower degree of curvature, into two intermediate zones of the belt. A description of an assembly forming such a rigid support is given in Great Britain Patent No. 1 000 094 to Woodhead. This patent shows that the central roller can be slightly offset, so that the central roller can be wider than the gap located between the oblique rollers.
Great Britain Patent No. 1 000 094 also discloses the use of flexible rollers in the form of coil springs connecting two roller bearings. Such rollers make possible a more gradual curvature which allows a belt of up to 1.80 meters wide to be used.
Such wide conveyors, however, have a tendency to lose the alignment between the flexible belt and the axis of the conveyor or the axis of the frame supporting the rigid supports or the flexible supports of the conveyor. Great Britain Patent No. 797 422 to Sutcliffe discloses a balancing device which equalizes the tension of flexible rollers. These flexible rollers consist of a flexible mandrel fastened to two bearings, with a connecting means, declared necessary, for the rotational connection of the flexible springs by universal joints, either between two rollers in a "V", or with the interposition of a horizontal roller at the center of the wide trough conveyor. Such a connecting device, however, essentially does not seem to have been generally adopted by other conveyor manufacturers.
When the rollers of a support remain rigid, a stable centering can essentially be obtained by a slight toe-in angle in relation to the vertical plane orthogonal to the belt. This toe-in gives the axis of the rollers, already inclined at 30 degrees from the horizontal, for example, an angle of 1.5 to 3 degrees from the vertical plane orthogonal to the belt.
Determined experimentally, this "toe-in" angle between vertical planes containing the rollers, induces a force toward the axis, which force is greater on the side which is more heavily loaded, tending to satisfy a trajectory in stable equilibrium when these components are balanced. An exaggerated positive toe-in, however, compresses the width of the belt, with a tendency to reduce the radius of curvature of the trough, and above all, results in a tendency to increase the resistance to the forward motion of the loaded belt, thus requiring excessive drive force to move the belt.
French Patent No. 2 267 961 to Valcalda, the inventor of the present application, combines the advantage of flexible rollers with a device designed to remedy the disadvantage of the negative toe-in exhibited under the action of the resistance to rolling. Because the rollers are driven, in an oblique plane, by the movement of the load, which movement causes instability under high load, the load capacity is limited.
Flexible rollers, one per support, make possible an economical catenary suspension of each of their external bearings, are driven in the direction of movement, and cause a negative toe-in, which causes instability of the conveyor belt. To make possible both high speeds and a deeper trough, which constitute the two requirements for a large flow capacity of material, French Patent No. 2 267 961 proposes the use of two flexible rollers per conveyor support, overlapped so that the asymmetrical toe-ins of each one are balanced out in a stable elastic recall for the loaded belt.
To do this, the internal stationary points of the two flexible rollers, which are necessarily overlapped (or supplemented by a third, horizontal, flexible roller, under the central portion of the load), are located significantly lower than the external fastening points. This French patent describes the possibility, since each flexible roller is itself no longer symmetrical in relation to the axis of the belt, of slightly shifting its own stationary points in the direction of movement, to produce, even before the appearance of the rolling resistance, a positive toe-in, which is a source of stability for the belt, as when the rollers are rigid.
The limitation to the benefit offered by the solution of French Patent No. 2 267 961 is that the stability in question, which is provided by the disclosed rollers when the belt is empty, increases further under the effect of a load, which load curves said flexible rollers. As a result, the toe-in effect, which is thereby increased, is accompanied by a prohibitive rolling resistance. It is therefore necessary to find an experimental compromise between the self-stability necessary primarily for the empty belt, e.g. when a great deal of tension is applied during start-up or when the loading begins, and an excessive stability when the belt is loaded, which load causes curvature of the flexible rollers. Under a load, and as a result of the increased curvature, the trajectory at each point of the rollers is therefore no longer strictly in the same direction, causing parasite frictions between the roller and the belt.
The prior art described above therefore leads to a contradiction between the ability to increase the width and the depth of the trough to improve the flow capacity of a conveyor, and the expenditure of the minimum energy necessary to guarantee the rolling required to reach satisfactory belt speeds.
In an attempt to preserve safety during empty phases of the conveyor operation, the effort to increase stability when the belt is both empty and under load, in effect, leads to an increase in energy consumption and in wear.